Electrostatographic machines generally use a two-component developer mixture comprised of a toner powder and a magnetized or magnetizable carrier material. During the use of the machine, toner powder must be replenished to compensate for its consumption during image development. Various automatic toner replenishment systems are known wherein a signal from a toner concentration monitor controls replenishment. Toner concentration monitors may take several forms, including optical sensors, capacitance sensors, resistance sensors, inductance sensors, magnetic sensors, etc.
Commonly assigned U.S. Pat. No. 4,797,704 which issued to L. A. Hill and M. E. Jacobs on Jan. 10, 1989, and U.S. patent application Ser. No. 215,971 filed by K. A. Arnold, L. A. Hill, and K. S. Robinson on July 7, 1988 disclose a replaceable development station for developing electrostatic images. The device is slid endwise into place in an electrostatographic machine so that the entire development station is removable when its original supply of toner is exhausted.
Generally, such development stations are disposable. Therefore it is not practical to provide a toner concentration monitor in each station. However, for toner monitors to work optimally, they must not be separated from the development mixture by thick development station walls. In commonly assigned U.S. patent application Ser. No. 258,938 filed by L. A. Hill, A. S. Kroll, and R. E. Williams on Oct. 17, 1988, a thinned region is defined in the wall of the development station such that the toner monitor is separated from the developer mixture by a wall region substantially thinner than the general wall thickness. The thinned region is formed by a recess in the wall of the station into which the toner monitor is urged when the station is slid into a receiving channel. The toner monitor is cammed away from the station as the development station is slid into the receiving means, and is resiliently urged into the recess when the development station is fully inserted in the receiving channel.